How to add dirt to a glass shader?
You can try adding a scratches map to your glass material. A quick Google search for "scratch bump map" can lead to some good results.

For the nodes, I would suggest two Glass BSDF shaders, one with 0 roughness and one with 0.4 roughness, run into a Mix Shader (0.4 Roughness one in top shader input). Use the image as a fac in the Mix Shader. You can add a Bright/Contrast node between the Image Texture and the Mix Shader. Adjust the Bright: value to affect the roughness.

Unfortunately, I can't post screenshots right now.